Programmed Data Processor (PDP) was a series of minicomputers made and marketed by the Digital Equipment Corporation from 1957 to 1990. The name 'PDP' intentionally avoided the use of the term 'computer' because, at the time of the first PDPs, computers had a reputation of being large, complicated, and expensive machines, and the venture capitalists behind Digital (especially Georges Doriot) would not support Digital's attempting to build a "computer"; the word "minicomputer" had not yet been coined.[citation needed] So instead, Digital used their existing line of logic modules to build a Programmed Data Processor and aimed it at a market which could not afford the larger computers.

The various PDP machines can generally be grouped into families based on word length.

18-bit machine intended to be a slower, cheaper alternative to the PDP-1; it was not considered commercially successful. All later 18-bit PDP machines (7, 9 and 15) were based on a similar, but enlarged instruction set, more powerful, but based on the same concepts as the 12-bit PDP5-PDP-8 series. One customer of these early PDP machines was Atomic Energy of Canada. The installation at Chalk River, Ontario included an early PDP-4 with a display system and a new PDP-5 as interface to the research reactor instrumentation and control.

PDP-5

DEC's first 12-bit machine. Introduced the instruction set later expanded from a to handle more bit rotations and a jup from a maximum 4K to a maximum 32K of core memory used in the PDP-8 - the first computer series with more than 1,000, then 10,000 built; Astronomical in the decade after ENIAC/UNIVAC builders predicted that 3 computers would serve the nations computing needs.

Replacement for the PDP-4; DEC's first wire-wrapped machine. The first version of Unix, and the first version of B, a predecessor of C, were written for this machine at Bell Labs, as was the first version (by DEC) of MUMPS.

12-bit machine with a tiny instruction set; DEC's first major commercial success and the start of the minicomputer revolution. Many were purchased (at discount prices, a DEC tradition, which also included free manuals for anyone who asked during the Ken Olsen years) by schools, university departments, and research laboratories. Later models were also used in the DECmateword processor and the VT-78workstation. It is reported that Edson de Castro, who had been a key member of the design team, left to form Data General when his design for a 16-bit successor to the PDP-8 was rejected in favour of the PDP-11; the "PDP-X" did not resemble the Data General Nova, although that is a common myth.[citation needed]

A hybrid of the LINC and PDP-8 computers; two instruction sets. Progenitor of the PDP-12.

PDP-9

Successor to the PDP-7, DEC's first micro-programmed machine. It featured a speed increase of approximately twice that of the PDP-7. The PDP-9 was also one of the first small or medium scale computers to have a keyboard monitor system based on DIGITAL's own small magnetic tape units (DECtape).[4] The PDP-9 established minicomputers as the leading edge of the computer industry.

The archetypal minicomputer; a 16-bit machine and another commercial success for DEC. The LSI-11 was a four-chip PDP-11 used primarily for embedded systems. The 32-bitVAX series was descended from the PDP-11, and early VAX models had a PDP-11 compatibility mode. The 16-bit PDP-11 instruction set has been very influential, with processors ranging from the Motorola 68000 to the Renesas H8 and Texas InstrumentsMSP430, inspired by its highly orthogonal, general-register oriented instruction set and rich addressing modes. The PDP-11 family was extremely long-lived, spanning 20 years and many different implementations and technologies.

A machine with 12-bit instructions, intended as an industrial controller (PLC). It had no data memory or data registers; instructions could test Boolean input signals, set or clear Boolean output signals, jump conditional or unconditionally, or call a subroutine. Later versions (for example, the PDP-14/30) were based on PDP-8 physical packaging technology. I/O was line voltage.

PDP-15

DEC's final 18-bit machine. It was its only 18-bit machine constructed from TTLintegrated circuits rather than discrete transistors, and, like every DEC 18-bit system (except mandatory on the PDP-1, absent on the PDP-4) had an optional integrated vector graphics terminal, DEC's first improvement on its early-designed 34n where n equalled the PDP's number. Later versions of the PDP-15 ran a real-time multi-user OS called "XVM". The final model, the PDP-15-76 used a small VAX-compatible to allow VAX peripherals to be used.[citation needed]

PDP-16

A "roll-your-own" sort of computer using Register Transfer Modules, mainly intended for industrial control systems with more capability than the PDP-14. The PDP-16/M was introduced as a standard version of the PDP-16.

TX-0 designed by MIT's Lincoln Laboratory, important as influence for DEC products including Ben Gurley's design for the PDP-1

LINC (Laboratory Instrument Computer), originally designed by MIT's Lincoln Laboratory, some built by DEC. Not in the PDP family, but important as progenitor of the PDP-12. The LINC and the PDP-8 can be considered the first minicomputers, and perhaps the first personal computers as well. The PDP-8 and PDP-11 were the most popular of the PDP series of machines. Digital never made a PDP-20, although the term was sometimes used for a PDP-10 running TOPS-20 (officially known as a DECSYSTEM-20).

Conversations with David M. Razler (dmrazler@razler.net), owner/restorer of PDP-7s,8s,9s and 15s until the cost of hauling around 2 tons of DEC gear led him to sell off or give away everything he owned.

Various sites list documents by Charles Lasner, the creator of the alt.sys.pdp8 discussion group, and related documents by various members of the alt.sys.pdp8 readership with even more authoritative information about the various models, especially detailed focus upon the various members of the PDP-8 "family" of computers both made and not made by DEC.